In recent years, a display device with a liquid crystal display (LCD) panel has become the mainstream for display devices. In such an LCD device, a thin film transistor (TFT) is formed on a glass substrate, and a backlight source is disposed on the rear side of an LCD panel in which liquid crystal elements are arranged. The backlight source emits light and the light goes through the panel, so that an image is shown on the display of the device. To obtain clear and bright images with high quality, the backlight source has to provide light with high brightness and uniformity on the display.
Conventionally, a cold-cathode tube has been used as a backlight source for a display device having an LCD panel. However, recent technical advances in light emitting diode (LED)s have drastically increased brightness and operating efficiency and catch up with the level almost equivalent to the cold-cathode tube. With the improvement as a backdrop, LEDs are becoming popular as a backlight source for a small-size LCD device.
However, a cold-cathode tube linearly emits light; on the other hand, an LED emits light in a dot shape when LEDs are used for a backlight source. Therefore, to achieve sufficient brightness as high as that of the cold-cathode tube, the structure of the light source has to be chosen from the two: using a backlight source having a plurality of low-output LEDs; or using a backlight source of a high-output LED with efficiently controlled light dispersion.
In terms of the cost and weight of products, the number of LEDs used for the light source has to be reduced as possible. On the other hand, decreasing the number of LEDs arises the need for increasing brightness of each LED, i.e., increasing input of each LED. This accordingly increases the temperature of the LED and the periphery of the LED. The increase in temperature can deteriorate reliability and operating life of not only the LED but also peripheral components.
To address the problem caused by heat, suggestions have been made in the following patent documents:    Japanese Unexamined Patent Application Publication No. 2002-229022; and,    Japanese Unexamined Patent Application Publication No. 2003-076287.
Focused on heat dissipation from LEDs, these suggestions introduce a structure where a metallic thin-film and a metallic frame, or a soft metallic sheet and a metallic frame are disposed so that the heat generated from LEDs is led to the metallic conductor having a broad area and dissipated from the surface thereof.
Besides, there has been a growing trend in recent years that a touch panel and a digitizer are mounted on data processing devices having the display device above. Serving as a man-machine interface, the touch panel and the digitizer add a function of detecting a position on the screen of the LCD panel. In the structure above, however, a problem arises. The digitizer, which is disposed on the back surface of the LCD panel, detects a position on the screen according to the movement of an electromagnetic pen operated on the front surface of the panel. When a metal including a heat-dissipation plate is disposed within the valid display area, the metal inconveniently affects the magnetic field between the digitizer and the pen, so that an accurate detection of an input position cannot be expected.